1. Field of the Invention
The present invention relates to an optical head used in magnetooptical memory system utilizing the magnetooptical effect to record and/or reproduce information in a magnetooptical record medium, and to an optical information reading apparatus for reproducing information in optical information record medium with reflected light of a light spot irradiating the medium.
2. Related Background Art
Recently active with the spread of computer is the development of a magnetooptical memory system capable of erasing or rewriting information as a large capacity external memory. Information may be recorded in a magnetooptical record medium by utilizing a local temperature rise in a magnetic thin film with irradiation of laser beam spot and reproduced by the magnetooptical effect, especially by the Kerr effect.
The conventional apparatus had a disadvantage of slow record time because of necessity of three operations in the information record process, i.e., erasure of already recorded or old information, record of new information, and verification of whether the new information is correctly recorded.
This problem may be dealt with for example by using the technique as described in Japanese Laid-open Patent Application No. 3-73448. The structure and function of the optical head as described in the above patent application is described below with FIG. 1.
In FIG. 1, a magnetooptical record medium 101 is driven to rotate in a certain direction while held on a turn table 102.
A laser beam emitted from a laser source 103 is collimated by a collimating lens 104, then passes through a beam splitter 105, and is converged by an objective lens 106 onto the magnetooptical record medium 101.
Magnetic field generating means 107 is disposed opposing to the laser beam irradiating portion on the back side of the magnetooptical record medium 101. The magnetic field generating means 107 generates a magnetic field changing depending upon information to be recorded, that is, an information-modulated magnetic field. The information-modulated magnetic field is applied to the laser beam irradiating portion on the magnetooptical record medium to effect the information record in the magnetooptical record medium 101.
The erasure operation of old information in a sector into which new information is to be written is carried out at the same time with the record operation. Namely, it is the magnetic field modulation type overwriting technique.
While the above overwriting operation is carried out, reflected light from the magnetooptical record medium 101 again passes through the objective lens 106, is then reflected by the beam splitter 105, passes through a half-wave plate 108, and thereafter is separated into two beams by a polarizing beam splitter 109.
One of the separated beams passes through a convex lens 111 and a cylindrical lens 112, and is received by a four-divided (4D) photodetector 113. The autofocus and autotracking operations are carried out based on calculation results with outputs from the 4D photodetector 113.
The other of the separated beams is converged by a convex lens 110 and is received by a photodetector 114. A magnetooptical signal may be detected from a differential output between a total output of the 4D photodetector 113 and an output of the photodetector 114. Namely, it is the direct verification technique. Accordingly, together with the overwriting technique, a one-pass writing operation is executed to simultaneously effect the erasure, the recording, and the verification.
FIG. 2 shows a state of a light spot on the magnetooptical record medium 101, in which the recording is carried out in the magnetic field modulation method and a state of magnetization in the magnetooptical record medium 101 in information record. The arrow in FIG. 2 represents a direction of relative movement of light spot to the magnetooptical record medium 101, and the right end circle represents a light spot. The right half area hatched in the light spot in FIG. 2 is a low-temperature region still insufficient in heating, which would be a region in which the inversion of magnetic field could be unstable in some record media.
The direct verification operation might not be carried out well by some chance in such record media. The detection system as shown in FIG. 3 is disclosed to overcome it.
In the conventional apparatus shown in FIG. 3, each of two photodetectors 131, 132 receives a reflected light beam from the magnetooptical record medium to generate a magnetooptical signal, which are two-divided (2D) photodetectors. In more detail, each photodetector 131, 132 has a photodetector segment 131a, 132a receiving a reflected light beam from the hatched region (see FIG. 2) insufficient in heating in the reflected light from the magnetooptical record medium, and a photodetector segment 131b, 132b receiving a reflected light beam from the region sufficient in heating in the reflected light. A reproduction magnetooptical signal (S.sub.6 +S.sub.7)-(S.sub.8 +S.sub.9) and a verification magnetooptical signal (S.sub.8 -S.sub.7) are obtained based on respective outputs S.sub.6, S.sub.9, S.sub.7, S.sub.8 from the above photodetector segments 131a, 132a, 131b, and 132b.
FIG. 4 shows another example of light spot, illustrating a state of a light spot on the magnetooptical record medium 101, in which the recording is carried out in the magnetic field modulation method, and a magnetization state in the magnetooptical record medium 101 in information record.
The arrow in FIG. 4 represents a direction of relative movement of light spot to the magnetooptical record medium 101, and the right end circle represents a light spot. A crescent area hatched in the light spot is a low-temperature region still insufficient in heating, which would be a region in which the inversion of magnetic field could be unstable in some record media. The aforementioned direct verification operation might not be carried out well by some chance in such record media. The detection system shown in FIG. 5 is disclosed to overcome it.
In the conventional apparatus shown in FIG. 5, each of two photodetectors 131, 132 receives a reflected light beam from the magnetooptical record medium to generate a magnetooptical signal, which are 2D photodetectors. In more detail, each photodetector 131, 132 has a photodetector segment 131a, 132a receiving a reflected light beam from the hatched low-temperature region (see FIG. 5) insufficient in heating in the reflected light from the magnetooptical record medium, and a photodetector segment 131b, 132b receiving a reflected light beam from a high-temperature region sufficient in heating. A reproduction magnetooptical signal (S.sub.6 +S.sub.7)-(S.sub.8 +S.sub.9) and a verification magnetooptical signal (S.sub.8 -S.sub.7) are obtained based on respective outputs S.sub.6, S.sub.9, S.sub.7, S.sub.8 from the photodetector segments 131a, 132a, 131b, and 132b.
Conventionally, the reading resolving power of an apparatus for optically reading information was determined by an optical spot size. The optical spot size is in proportion to the wavelength of light as well as to the inverse of angular aperture of condenser lens. A reduction in wavelength of light is restricted by structural issues of light source, for example of a semiconductor laser, while an increase in angular aperture of condenser lens has a designing limit.
In order to obtain a signal of smaller area than the optical spot, it is conceivable that the photosensor is divided into plural segments.
FIG. 6 shows a constitutional example of a conventional optical information reading apparatus. A laser beam emitted from a semiconductor laser 601 passes through a lens 602, a beam splitter 603 and a lens 604, and is condensed at the point of 605 to irradiate a record medium 606. Light reflected from the medium 606 passes through the lens 604, the beam splitter 603 and a lens 607 to irradiate a sensor 608. The sensor 608 is divided into two segments 608a and 608b.
Now described with FIG. 7 is a relation between the spot 605 and an image on the sensor 608 in this arrangement. FIG. 7 is simplified by omitting the parallel beam portion between the lens 604 and the lens 607 in FIG. 6. In FIG. 7, if a lens 610 with focal length f is placed to a spot 609, an image of the spot 609 is formed at a point 611, macroscopically satisfying the relation, (1/a)+(1/b)=1/f, and the image of spot 609 may be projected onto a sensor 611 placed there. Thus, light from segment 609a reaches a region 611a while light from a segment 609b reaches a region 611b. Therefore, returning to FIG. 6, information in the region 605a on the record medium 606 must reach the sensor segment 608b while information in the region 605b the sensor segment 608a.
Actually, as the spot becomes smaller, the border line becomes unclear between the regions 605a and 605b projected onto the sensor 608 due to the aberrations of lens and the diffraction of light wave. Since the image projected onto the sensor 608 decreases with smaller spot, it becomes difficult to make the border between the regions 605a and 605b in the image coincident with the border between the sensor segments 608a and 608b. Further, the image might be buried in the insensitive zone between the sensor segments 608a and 608b.
Then, the sensor 608 in FIG. 6 is actually arranged as to be deviated from the image-formed position. If the sensor 608 is deviated before the image-formed position, the image is blurred, but more reflected light from the region 605a on the spot enters the sensor segment 608a and more reflected light from the region 605b enters the sensor segment 608b because of the diffraction. However, their separation would be insufficient.
In the above-described conventional examples, however, a drawback is an increase in size of optical head, because a convex lens 110 and a photodetector 131, 132 comprised of two segments must be provided for each beam separated by the polarizing beam splitter 109, specifically as shown in FIG. 3.
Another disadvantage is an increase in production cost due to the increased number of components. Further, the photodetectors 131, 132 must be aligned with the respective beams independently of each other, resulting in spending a lot of time for position alignment.